1. Field of the Invention
The present invention relates to a wireless communication device for performing bi-directional communication with a remote communication device while repeatedly hopping a carrier wave frequency at a predetermined hopping pattern.
2. Description of Related Art
Recently, there has been proposed a wireless communication device of a spread spectrum type. In order to transmit data to a remote communication device, the communication device modulates data into a modulation signal and transmits the modulation signal with a carrier wave whose frequency is widely spread. Upon receipt of the signal, the remote communication device despreads the frequency of the carrier wave to obtain the modulation signal, demodulates the signal into the original data. According to this spread spectrum method, it is possible to perform bi-directional communication with a plurality of frequencies with low power density.
A frequency hopping method is one of the spread spectrum method. According to the frequency hopping method, when transmitting data, the data is first modulated into a modulation signal. The frequency of the modulation signal is then spread into a wider range through repeatedly multiplying the modulation signal with a signal with a repeatedly-hopping frequency. The thus frequency-spread signals are transmitted to the remote communication device. In other words, the modulation signals are transmitted with a frequency-hopping carrier wave. Upon receipt of the modulation signals, the remote communication device first despreads the frequency of the received signals into the original frequency through multiplying the received signal with the signal of the repeatedly-hopping frequency. Then, the frequency-despread signals are demodulated into the original data.
According to the frequency hopping method, it is possible to reliably protect the privacy. It is also possible to decrease the possibility that a communication trouble will occur due to interference with other electric signals. The frequency hopping method is therefore being widely used in telephones, facsimile machines, and the like.
The wireless communication device, for performing the frequency hopping communication, is conventionally provided with a hopping pattern table. The hopping pattern table stores therein a predetermined number of sets of predetermined hopping frequency data which are indicative of the predetermined number of frequencies to be hopped. The communication device therefore performs communication with a carrier wave whose frequency is hopped according to the hopping pattern stored in the hopping table.
There is a possibility, however, that a communication device operating in a reception mode is located near some electric sources which are outputting electric signals with high power. In this case, interference occurs between the electric signals (communication disturbing signals) and the frequency-hopping carrier wave signals at some frequencies. The communication device may not properly demodulate the received signals into the original data. In other words, communication trouble occurs due to the interference between the frequency-hopping carrier wave signals and the communication disturbing signals. Reliability in the communication is deteriorated.
Japanese Patent Application Publication (Laid-Open) No. 6-334630 provides a method for previously measuring the received level of the communication disturbing signals at a plurality of different frequencies. The predetermined number of frequencies are then selected based on the measured levels. That is, a frequency, that provided the smallest level, is selected first. Then, another frequency, that provided the second smallest level, is selected second. Thus, the predetermined number of frequencies are successively selected in the order of their measured levels. Thus, a hopping patter table, storing the predetermined number of frequencies, is created. The thus created hopping pattern table is provided to each of all the communication devices in the same communication group. Accordingly, communication will be performed only with those hopping carrier waves whose frequencies are confirmed to develop low degree of interference with communication disturbing signals. The reliability in communication is therefore greatly enchanced.
Japanese Patent Application Publication No.7-107010 provides another method for enabling communication with only those carrier waves that do not suffer from any communication troubles. According to this method, the communication device continuously monitors a reception state of signals transmitted from a remote communication device. When detecting that the reception state is deteriorated or that an error rate is high at a certain frequency due to interference with communication disturbing signals, the communication device deletes the corresponding frequency data from the predetermined hopping pattern table. The communication device also adds the hopping pattern table with new frequency data which is not yet used and which is confirmed not to interfere with any communication disturbing signals. Thus, when communication starts, the hopping pattern is successively changed at the communication device of the receiving side. At each communication blank time for hopping frequency from one to another, information on the changed hopping pattern is transmitted from the communication device at the reception mode to the remote communication device at the transmission mode.